1. Field of the Invention
The present invention relates to a method of gettering an impurity element that is contained in a semiconductor film. Also, the present invention relates to a technique of manufacturing a highly reliable semiconductor device of excellent characteristics from a crystalline semiconductor film that is formed in accordance with the present invention.
2. Description of the Related Art
In order to improve the yield in manufacture of elements that utilize semiconductor characteristics, for example, thin film transistors (TFTs) and like other devices, and to make the devices more minute and sophisticated, it is important to develop a gettering technique for lowering the concentration of a heavy metal impurity element in a semiconductor film (a technique for moving an impurity element in a semiconductor film to a gettering region through diffusion induced by heat treatment). Ways to make devices smaller and improve their performance are constantly being researched and, as the size and weight are reduced, influence of a heavy metal impurity element inversely becomes larger. As a result, even a small amount of heavy metal impurity element in a device causes problems such as defects and leak current to degrade characteristics of the device and shorten the lifetime of the device.
Accordingly, technical development related to the gettering technique for reducing the impurity element concentration in a semiconductor is being actively advanced in order to improve the yield and obtain a high performance device.
The applicants of the present invention have disclosed in JP 07-161634 A and other published articles a method to obtain a quality crystalline semiconductor film. In this method, an amorphous semiconductor film (typically an amorphous silicon film) is doped with a metal element such as Ni, Cu, or Pd, and then subjected to heat treatment to form a quality crystalline silicon film that is large in grain size. The metal element used to dope the amorphous silicon film is called an element that accelerates crystallization or a catalytic element since its role is to accelerate crystallization. Ni given here as an example of the catalytic element (element that accelerates crystallization) is also one of the above-described heavy metal impurity elements that cause degradation of characteristics of semiconductors.
Therefore, the catalytic element should be removed from the crystalline semiconductor film (crystalline silicon film) (or the concentration of the catalytic element (element that accelerates crystallization) in the crystalline silicon film should be lowered) quickly after the crystallization step. Various gettering techniques have been devised to remove or reduce the catalytic element.
Techniques for gettering an impurity element (element that accelerates crystallization) can be divided by their gettering mechanisms into three types. Type 1 refers to those that make the impurity element diffuse, by heating, from a region that later serves as a channel formation region to a source region or drain region that contains an element having a gettering action (for example, an element belonging to Group 15 in the periodic table) in high concentration. Type 2 refers to those that make the impurity element diffuse, by heating, from an active layer region (especially a region that later serves as a channel formation region) to a gettering region which is outside of the active layer region and which contains an element having a gettering action (for example, an element belonging to Group 15 in the periodic table) in high concentration. Type 3 refers to those that make the impurity element diffuse, by heat treatment, from a first semiconductor film (a semiconductor film for forming an active layer) to a second semiconductor film (for example, a silicon film) that is formed on the first semiconductor film to serve as a gettering region.
Especially, Type 3, which moves the impurity element in the direction substantially perpendicular to the substrate for gettering (hereinafter referred to as vertical gettering), is deemed as a promising method to obtain a quality semiconductor film with low impurity element concentration because the distance the impurity element moves is short and this is advantageous in lowering the impurity element concentration in the semiconductor film satisfactorily.
JP 10-022289 A discloses a method of gettering an impurity element contained in a semiconductor film by forming an oxide film with a thickness of 1 to 5 nm on the semiconductor film, forming a film that contains silicon on the oxide film, and subjecting the film to heat treatment.
The oxide film on the semiconductor film functions as an etching stopper (etching block) when the gettering region is removed by etching after the heat treatment for gettering. For that reason, the oxide film is required to “withstand well against etching and protect the semiconductor film from an etchant as an etching stopper”, “allow the impurity element to move in itself during heat treatment for gettering”, and “have excellent reproducibility”.
However, how the oxide film that meets the above requirements can be obtained is not mentioned in JP 10-022289 A although the publication states that an oxide film for effective gettering is 1 to 5 nm in thickness.